xc2xa71.1 Field of the Invention
The present invention concerns methods and apparatus for terminating a line and for supporting the asymmetric digital subscriber line (or xe2x80x9cADSLxe2x80x9d) protocol. More specifically, the present invention concerns a digital access arrangement for performing line isolation, transmit/receive signal separation, and/or echo cancellation functions.
xc2xa71.2 Related Art
As set forth in detail below, the ADSL protocol is believed to be an important technique for data communications, particularly for data communications needed to provide video-on-demand services.
xc2xa71.2.1 Unmet Demand for Telecommuting and Video-On-Demand Services
In the near future, it is believed that applications for providing video-on-demand and for facilitating telecommuting will be developed in order to meet perceived demand for such services. First, regarding video-on-demand, current techniques for delivering video entertainment, such as movies, to consumers include television broadcasts, cable services, and video tape and disk rentals. Though television broadcasts and cable services deliver video entertainment to customers"" homes, such services are limited in that customers must view such entertainment at a time dictated by a fixed schedulexe2x80x94not necessarily when they want to view such entertainment. Further, VCR type functions such as pause, stop, fast forward, and rewind, are not available.
Although video recorders permit customers to record video programs for later viewing at a convenient time, customers are often put off by the task of setting such video recorders. On the other hand, video tape and disk rentals permit customers to watch a particular movie or program at a time convenient. However, such movies or video programs are not delivered to their homexe2x80x94the customer must pick up and drop off the video tapes or disks. Thus, a service that provides video entertainment to customers (i) at their residences and (ii) when they want it would serve a strong, yet unmet, demand.
xc2xa71.2.2 Transmission Facilities to Customer Premises and Their Limitations
Video data can be transmitted to a customer""s premises by a physical transmission medium, such as co-axial or fiber optic cable for example, or by a wireless method, such as television broadcast or satellite transmission for example. The limitations of each of these broad categories is discussed below.
The problem with using physical transmission medium is, to the extent not already facilitated by existing plant (e.g., telephone lines, co-axial cable, etc.), such physical transmission medium must be provided to the customer""s premises. Installing new transmission medium to the premises of many customers (also referred to as xe2x80x9clast milexe2x80x9d transmission) entails tremendous costs. Thus, to the extent that existing infrastructure, and in particular, existing physical transmission medium, exists, such existing infrastructure should be exploited.
Presently, the most prevalent physical transmission media entering customer""s premises are (a) twisted pair copper wire (also referred to as xe2x80x9ctwisted pairxe2x80x9d) and (b) coaxial and/or hybrid fiber-coaxial (or xe2x80x9cHFCxe2x80x9d) cable. Twisted pair has been used, traditionally, for voice telephone services, and more recently, for data communication by means of modems. Some believe that the use of twisted pair for video-on-demand and telecommuting applications has advantages over the use of coaxial cable or hybrid fiber-coaxial cable for the reasons discussed below. In the following, ADSL is a communications protocol supported over twisted pair. Cable modems are used to communicate data over coaxial or hybrid fiber-coaxial cable.
Installed infrastructure presents the largest advantage of ADSL over cable modems. In 1998, the global ratio of telephone lines to HFC lines is about 400 million to 6 million, or about 60 to 1. Aggressive upgrades from coaxial cable to HFC cable over the next five. (5) to six (6) years will not improve the ratio to better than 10 to 1. Even in the United States, the ratio of telephone lines to HFC lines is on the order of 20 to 1. Based on estimates of the International Telecommunications Union (or xe2x80x9cITUxe2x80x9d), in 1995, 700 million telephone lines existed, about 500 million of which served residences and the balance serving businesses and pay telephones.
Regarding line quality, cable modems have the advantage that they do not depend on coaxial cable distance. This is because amplifiers in the cable network boost signal power sufficiently. Unfortunately, however, most existing CATC systems are not HFC, but rather, tree and branch networks of coaxial cable. These networks use one-way amplifiers that preclude upstream data flow. Since 1993, many CATV lines have been installed with two-way amplifiers creating an upstream data path from 5 to 45 MHz. However, the sheer size of these networks (sometimes, as many as 10,000 customers may be served from a single headend) and the noise and channel problems with so many subscribers attached to a common line, make high speed upstream channels unattainable after a few subscribers have joined the line. A unidirectional coaxial CATV network may be upgraded to a bi-directional one by physically replacing amplifiers, at a cost of around $25 per home passed. Upgrading from coaxial to HFC cable requires more work and costs.
On the other hand, ADSL modem speeds will depend on line distance. The longer telephone lines found today may support speeds no greater than 1.5 Mbps. However, the average telephone line will support speeds up to 6 Mbps. Variable rate ADSL modems will be able to adapt their rate to line length.
Finally, telephone networks have historically been more reliable and stable than coaxial cable networks. Thus, some believe that ADSL, rather than cable modems, will be the preferred means of providing video-on-demand services.
Although wireless video transmission methods, such as satellite for example, do not suffer from the xe2x80x9clast milexe2x80x9d problem of wiring to each customer""s premises, the transmission is one wayxe2x80x94from the service provider to the customer. There is no backchannel communications path from the customer""s premises to the service provider. User commands are to a tuner at the customer""s premises, not to the service provider. Thus, video-on-demand and VCR type functions are not supported by satellite television systems.
xc2xa71.2.3 Overview of ADSL
Having described the relative advantages of ADSL for providing video-on-demand services for example, a technical description of ADSL, known to those skilled in the art, is provided in xc2xa71.2.3.1 below for the reader""s convenience.
xc2xa71.2.3.1 Technical Description of ADSL
xc2xa71.2.3.1.1 Data Rates
Recall that video-on-demand is believed to be a significant, yet un-served, market. Present video and audio data compression standards, such as the MPEG-2 (i.e., motion pictures expert group) standard, permit full motion video to be represented by a data stream having a rate of about three (3) Mbps. Providing back channel commands, such as program selection and menu navigation, VCR type commands, etc., obviously requires much less bandwidth. ADSL provides a downstream (i.e., from a service provider to a customer) data rate of 0.5 to 8 MBPS, an upstream (i.e., a back channel from the customer to the service provider) data rate of 64 to 640 KBPS, and traditional telephone service (also referred to as xe2x80x9cPOTSxe2x80x9d or xe2x80x9cplain old telephone service).
As a practical matter, the length of the local loop, that is the length of the twisted pair copper wire from a customer premises to a central office or other network node, as well as the gauge of the copper wire used in the twisted pair, limits the downstream data rate of ADSL. For 24 gauge (i.e., 5 mm cross section) wire, the downstream data rate limits are as follows:
Downstream data rates for 26 gauge (i.e., 4 mm cross section) wire are slightly less.
As discussed above, variable rate ADSL modems will be able to throttle the data rate based on the condition (e.g., length, gauge, etc.) of the local loop serving the customer""s premises.
xc2xa71.2.3.1.2 Modulation Frequencies
The frequency band of the telephone network was historically limited, by low pass filters at the fringes of the network, from 0 to 3.3 KHz. ADSL requires that these filters be removed. Frequency division multiplexing (or xe2x80x9cFDMxe2x80x9d) is then used to separate the frequency band of the twisted pair into a band for downstream data, a band for upstream data, and a band for POTS. As shown in FIG. 1A, a POTS channel is provided from 0 to 4 KHz, while a POTS guardband extends to 25 KHz. The upstream data channel is provided from about 25 KHz to about 100 KHz. Finally, the downstream data channel is provided from about 120 KHz to about 1.142 MHz. Also, as shown in FIG. 1A, the amplitude of the upstream and downstream signals is +20 dBm. The downstream data channel may be time division multiplexed (TDM) into one or more high and low speed channels. As shown in FIG. 1B, if echo cancellation is provided, the upstream data channel can extend to about 138 KHz such that it partially overlaps the frequency band of the downstream data channel.
xc2xa71.2.3.1.3 Modulation Techniques
Two alternative, and incompatible, modulation techniques have been competing; namely, carrierless amplitude phase (or xe2x80x9cCAPxe2x80x9d) modulation and discrete multitone (or xe2x80x9cDMTxe2x80x9d) modulation. Each is briefly discussed below.
CAP modulation is a variant of quadrature amplitude modulation (or xe2x80x9cQAMxe2x80x9d) which is the scheme used in most voice grade modems following the V.32 standard. Basically, CAP modulation uses both multilevel amplitude modulation and phase modulation and initially transmits a xe2x80x9ccarrierlessxe2x80x9d signal. The CAP modulation system may use a number of different error correction schemes such as: (i) Reed Solomon forward error correction (or xe2x80x9cFECxe2x80x9d) in the downstream direction for improving reliability in the event of impulse noise; (ii) an interleaving technique to reduce block error; (iii) Trellis encoding, typically provided in the upstream direction, for minimizing cross-talk, background, and white noise. Since each of these error correction techniques are known to those skilled in the art and not particularly relevant to the present invention, they are not described in further detail. It suffices to note that the forward error correction schemes are important in time sensitive (real time) data applications, such as video-on-demand for example.
DMT modulation divides the bandwidth of the copper line twisted pair into 256, 4 KHz wide, sub-channels, referred to as xe2x80x9cbinsxe2x80x9d. These bins are creating by using the fast Fourier transform (or xe2x80x9cFFTxe2x80x9d) at the receiver and inverse fast Fourier transform (or xe2x80x9cIFFTxe2x80x9d) at the transmitter. DMT allocates data to the bins based on the noise then being experienced in each bin. The digital signals may be encoded with error-correcting codes, for dealing with occasional bursts of impulse noise, similar to those employed on compact disks.
xc2xa71.2.4 Exemplary Environment in Which the Present Invention May Operate
FIG. 2 is block diagram which depicts an environment 200 in which the present invention may operate. This environment is similar to the ADSL Forum System Reference Model, set forth in Annex A of Technical Report TR-007, entitled xe2x80x9cInterfaces and System Configurations for ADSL: Customer Premisesxe2x80x9d (March 1988). As shown, at a high level, a customer premises 220 is coupled with an access node (such as a central office of a local telephone service provider) 210 via a local loop 230 comprising a twisted pair copper wire. Recall that the data rates which can be supported over the local loop 230 will be limited by the gauge and length of the local loop 230.
At the access node 210, a splitter 212 separates the POTS frequency band from the data frequency bands. Referring back to FIGS. 1A and 1B, the splitter 212 may include a low pass filter for passing frequencies from 0 to 25 KHz to a line (expanded) side input of a voice traffic switch 218. The splitter 212 may also include a high pass filter for passing frequencies above 25 KHz to an ADSL transmission unit 214. The ADSL transmission unit 214 may be coupled to an expanded side input of a data traffic switch or router 216.
A voice network 250, such as the public switched telephone network (or xe2x80x9cPSTNxe2x80x9d) for example, may connected, via one or more trunks 219, with a trunk (concentrated) side input of the voice traffic switch 218. A data network 240, which may also be the PSTN for example, may be connected, via one or more lines 217, with a concentrated side input of the data traffic switch or router 216.
The customer premises 220 may include POTS equipment 224 (such as a telephone and/or a modem for example) and a terminal 228 (such as a personal computer and/or a set top box for example). Like the splitter 212 at the access node 210, a voice/data splitter 222 separates the POTS frequency band from the data frequency bands. Once again, referring back to FIGS. 1A and 1B, the splitter 222 may include a low pass filter for passing frequencies from 0 to 25 KHz to the POTS equipment 224. The splitter 222 may also include a high pass filter for passing frequencies above 25 KHz to an ADSL transmission unit (which may also be referred to as a xe2x80x9cterminal adapterxe2x80x9d) 226. The ADSL transmission unit 226 may be coupled with the terminal 228. Although not shown in FIG. 2, the functions of the voice data splitter 222 may be incorporated into, or provided downstream from, the digital access arrangement 260.
The ADSL transmission unit 226 at the customer premises 220 may include a digital access arrangement (or xe2x80x9cDAAxe2x80x9d) 260, a digital signal processor (or xe2x80x9cDSPxe2x80x9d) 270, and a controller 280. The present invention concerns the digital access arrangement 260. The digital access arrangement 260 basically functions to: (i) isolate the downstream components from the twisted pair copper wire 230; and (ii) separate the upstream and downstream communications channels, for example, by hybrid splitting, transmit/receive signal filtering, and/or echo cancellation. The digital signal processor 270 basically functions to: (i) convert digital signals to analog signals (or xe2x80x9cDACxe2x80x9d) and analog signals to digital signals (or xe2x80x9cADCxe2x80x9d); (ii) modulate outgoing (i.e., upstream) signals and demodulate incoming (i.e., downstream) signals in accordance with either the CAP or DMT modulation methods; and/or (iii) cancel echo. Finally, the controller 280 basically functions to throttle outgoing (i.e., upstream) data rates to 64 Kbps to 640 Kbps and to buffer the incoming (i.e., downstream) data stream so that it may be provided at a data rate compatible with the terminal 228.
xc2xa71.2.5.1 Challenges to the Digital Access Arrangement
As introduced above, the digital access arrangement 260 basically functions to (i) isolate the downstream components from the twisted pair copper wire 230; and (ii) separate the upstream and downstream communications channels. Each of these functions, and challenges associated with performing these functions, will be addressed below.
xc2xa71.2.5.1.1 Line Isoloation
The twisted pair copper wire 230 may be terminated at the digital access arrangement 260. The twisted pair copper wire 230 includes wires known as a xe2x80x9ctipxe2x80x9d wire and a xe2x80x9cringxe2x80x9d wire. To protect (for example, in the event of lightening striking the local loop or a power line crossing the local loop) a customer and equipment at their premises, the digital access arrangement 260 isolates the twisted pair copper wire 230 from downstream equipment. Traditionally, transformers were used to inductively isolate the tip and ring lines from downstream equipment, such as telephones for example. As shown in FIGS. 1A and 1B, the POTS band is at a lower frequency (i.e., 0 to 4 KHz) and POTS uses a lower power than the upstream and downstream data in ADSL. At the higher frequencies and power used for upstream and downstream data in ADSL, the magnetic cores of transformers must be designed large enough so that they do not saturate or operate in a non-linear region.
This general drawback of using transformers for inductive line isolation actually precludes the use of transformers in certain applications. For example, PCMCIA (or personal computer memory card international association) cards used in portable computers, such as laptop computers and hand-held computers cannot use, practically, transformers for isolating ADSL lines. Thus, a digital access arrangement which provides line isolation with smaller and lighter components is needed.
Optical components are relatively small and light components and have been used by modems for line isolation. More specifically, a photo-transmitter (e.g., a light emitting diode, photo diode, photo transistor, etc.) provides a line signal to an adjacent photo-receptor. The receptor generates a voltage based on the state of the adjacent transmitter. Thus, the optical isolators used in modems operate in a xe2x80x9cphoto-voltaicxe2x80x9d mode. In addition, the optical isolators used in modems operate in a binary (i.e., either ON or OFF) mode. Unfortunately, these optical components, operating a photo-voltaic mode, cannot provide line isolation in relatively high data rate applications.
xc2xa71.2.5.1.2 Transmit/Receive Signal Separation
Recall that the tip and ring copper lines of the twisted pair 230 can simultaneously carry both upstream data and downstream data (in addition to POTS which may have already been filtered out). Referring back to FIG. 1A, if frequency division multiplexing (or xe2x80x9cFDMxe2x80x9d) is used, the upstream and downstream data channels can be easily separated by separating, by appropriate filtering for example, the upstream frequency band (e.g., 25 KHz to 100 KHz) from the downstream frequency band (e.g., 120 KHz to 1.142 MHz). Referring back to FIG. 1B, if the frequency bands of the upstream and downstream data channels overlap to some extent, echo cancellation may be used to separate the upstream and downstream data.
The present invention provides a method for transmitting and receiving data over tip and ring lines. Data may be transmitted by (i) buffering data received from an external source to generate buffered data, (ii) modulating the buffered data in accordance with an ADSL modulation technique (such as CAP or DMT) to generate modulated data, (iii) converting the modulated data to an analog signal, (v) amplifying the analog signal, while passing it over an optical isolation boundary, to generate an amplified analog signal, (v) selectively filtering the amplified analog signal to generate an amplified and filtered analog signal, and (vi) applying the amplified and filtered analog signal to the tip and ring lines. Data may be received by (i) taking a signal appearing across the tip and ring lines, (ii) blocking a DC component of the signal, to generate a non-biased signal, (iii) amplifying the non-biased signal to generate an amplified signal, (iv) converting the amplified signal to a digital signal, and (v) demodulating the digital signal in accordance with an ADSL demodulation technique (such as CAP or DMT).
When selectively filtering the amplified analog signal, frequencies below 25 Khz may be filtered out. Further, frequencies above 100 Khz (or 138 KHz) may also be filtered out.
The analog signal may be amplified by (i) applying the analog signal to a light emitting element, and (ii) receiving an amplified signal, as a current, at a light receiving element. A feedback signal, generated by a second light receiving element, may be applied to the analog signal.
The present invention also provides a device for terminating tip and ring lines. The device may include a line interface stage, a transmission stage, and a reception stage. The line interface stage may be coupled with the tip and ring lines, and may be used to separate, at least to some extent, transmit and receive signals on the tip and ring lines. The transmission stage may be arranged between the line interface stage and a data source. It may include an optical isolation unit for electrically isolating the data source from the tip and ring lines and for amplifying a signal received from the data source. The amplified signal is then provided to the line interface stage. The optical isolation unit of the transmission stage may operate in a photo conductive mode. The reception stage may be arranged between the line interface stage and a data sink. It may include capacitors for electrically isolating the data sink from the tip and ring lines.
The line interface stage may filter signals from the transmission stage to remove frequencies below 25 KHz. The line interface stage may also filter signals from the transmission stage to remove frequencies above 100 KHz (or 138 KHz).